1. Technical Field
This invention relates to telephone set voice circuits and, more particularly, to voice circuits for electronic telephone sets employing active speech networks that are powered from a central office line.
2. Description of the Prior Art
Two wire subscriber circuits in telephone systems are particularly arranged in hybrid type configuration with the transmitter and receiver unit mutually conjugate. Because of this arrangement, signals generated either in a transmitter or receiver are substantially or entirely muted in the other. This arrangement has traditionally been accomplished with a hybrid induction coil. One-half of the signal power generated by the transmitter, however, is dissipated in the balancing network of the hybrid induction coil resulting in low transmitting efficiency.
As a result of this power loss, as well as the undesirably large bulk and high cost of telephone set hybrid induction coils, voice circuits that avoid the conventional need for induction hybrid coils have been devised. One example of such a circuit is shown in U.S. Pat. No. 3,546,395, issued to P. O. Schuh on Dec. 8, 1970. The need for inductive hybrid coils is avoided through a circuit that employs the feedback loop of a transistor amplifier to provide sidetone balance.
In connection with the development of an all-electronic telephone set, it is desirable to provide an active speech network for use in these telephones that allows parallel operation with conventional hybrid coil telephones on long loop lengths. Current consumption for the active speech network must therefore be low to allow sufficient current for operation of the hybrid coil telephone. Another consideration is the power supply constraints from other electronic circuits within the electronic telephone. Circuits, such as an electronic dial and a linear transducer for the transmitter, require that the speech network maintain a substantially constant network tip-to-ring voltage and equalize transmission based on network loop current.